1. Field of the Invention
The present invention is a detection system that may be used to continuously determine the position of an object with respect to a magnetic field source. More particularly, this detection system determines the distance of the object from the magnetic field source by extrapolating and comparing magnetic field data generated by multiple magnetic field sensors spaced at known distances from one another.
2. Description of Related Art
Numerous systems have been proposed for coupling a magnetically coded signal to an object to be guided, such as a vehicle traversing a roadway. These systems rely on an array of magnetic fields generated by permanent magnets embedded in or placed atop the roadway. A transducer on the object to be guided derives an electric signal in response to the magnetic field signal from the roadway. Most of these systems include transducers responsive to the magnitude or polarity of the magnetic field. However, since the magnitude of the magnetic field is constantly varying as a result of noise and changing environmental conditions, few of these guidance systems have been able to reliably guide an object along a roadway.
In some of these systems, a single magnetic sensor on the object to be guided measures all components of the guiding magnetic field at a single location, and then utilizes these data to guide the vehicle. This approach requires complex processing hardware and is quite expensive. In addition, if the height of the magnetic sensor varies with respect to the magnetic field source, if the location of the magnetic sensor on the object changes, or if the output of the magnetic field source varies for any reason, the distance information generated by the system becomes unreliable. These systems are also greatly affected by noise, such as the magnetic signals produced by metallic objects. Some systems have attempted to solve these problems by storing and accessing previously measured height and magnetization information. However, if conditions change, the assumptions under which these height and magnetization data were generated are no longer applicable, and system performance degrades.
Other systems compare the electrical signals produced by two transducers and feed these data back to a guidance system to maintain the position of the object centered directly over the magnetic field source. These systems are useful for objects that travel slowly under controlled conditions, but if the object crosses over the magnetic field source and is not centered over the source, system performance degrades.
This invention provides a simple and inexpensive system for determining the position of an object relative to a magnetic field source. This system determines the position of an object independent of magnetic field source magnetization information and independent of the height of the magnetic field sensors relative to the magnetic field source.
In one embodiment, the present invention is a detection system for determining the position of an object as the object moves along a first direction. The system of the invention includes a magnetic field source generating a magnetic. field signal and a magnetic field detection system coupled to the object. The detection system includes a source interface module with magnetic field sensors positioned a known distance apart along a second direction different from the first direction. Each sensor detects the magnetic field generated by the magnetic field source and generates a magnetic field signal corresponding to the relative intensity of the magnetic field detected by the sensor. A processing module processes the magnetic field signals produced by the source interface module. Using the magnetic field signals, the processing module first determines a magnetic field peak along the first direction for each sensor. The processing module then compares the magnetic field peak data to determine the distance of the object from the magnetic field source along the second direction.
In a second embodiment, the present invention is a detection system for determining the position of a vehicle as the vehicle travels in a first direction. The detection system determines the position of the vehicle along a second direction substantially normal to the first direction. A magnetic tape mounted on the surface along the first direction is used as a magnetic field source. The tape generates an oscillating magnetic field signal in the first direction and in a second direction substantially normal to the first direction. The magnetic field detection system is coupled to the vehicle, and includes a source interface module with a centrally located magnetic field sensor, at least one first magnetic field sensor located on a first side of the central sensor, and at least one second magnetic field sensor located on a second side of the central sensor opposite the first side. The magnetic field sensors are aligned with one another and positioned a known distance apart along the second direction. Each sensor detects the magnetic field generated by the tape in the first direction and generates a magnetic field signal corresponding to the relative intensity of the magnetic field detected by the sensor. A processing module processes the magnetic field signals produced by the source interface module. The processing module first determines from the magnetic field signals a magnetic field peak in the first direction for each sensor. The processing module then uses the magnetic field peaks to calculate a first slope of a first line between a data point for the centrally located magnetic field sensor and a data point for at least one first magnetic field sensor located on the first side of the centrally located magnetic field sensor. The processing module then calculates a second slope of a second line between the data point for the centrally located magnetic field sensor and a data point for at least one peripheral magnetic field sensor on the second side. Then the processing module evaluates the first slope, the second slope and the peak data to determine the distance of the object from the tape along the second direction.
In a third embodiment, the present invention is a position detection module that may be detachably mounted on a vehicle moving in a first direction. The module is typically encased within an elongate enclosure. Within the enclosure are at least two magnetic field sensors. The sensors are aligned with one another along a second direction substantially normal to the first direction. Each sensor detects the magnetic field generated by the tape in the first direction and generates a magnetic field signal corresponding to the relative intensity of the magnetic field detected by the sensor. A processing module processes the magnetic field signals produced by the magnetic field sensors to determine a magnetic field peak in the first direction for each sensor. The processing module then evaluates the magnetic field peaks using a known calibrated slope to determine the distance of the object from the tape along the second direction.
In a fourth embodiment, the present invention is a method for determining the position of an object that is moving in a first direction. The method includes the initial step of generating a magnetic field signal in the first direction. This magnetic field signal is then sensed with an array of magnetic field sensors positioned a known distance apart along a second direction different from the first direction. Each sensor in the array detects the magnetic field signal along the first direction and generates a magnetic field signal corresponding to the relative intensity of the detected field. The magnetic field signals are used to determine a magnetic field peak in the first direction for each sensor. The magnetic field peaks from the sensors in the array are used to calculate a first slope of a first line between a first data point for a first sensor and a second data point for a second sensor. The peaks are next used to calculate a second slope of a second line between the first data point for the first sensor and a third data point for at least one third sensor. The processing module then evaluates the first slope, the second slope, and the peak data using the calibrated slope to determine the distance of the object from the tape along the second direction.
In a fifth embodiment, the present invention is a computer readable article of manufacture containing program code that, when executed by a processor, causes a digital computer to perform a series of operations. The computer receives input signals representative of magnetic field signals generated by each sensor in an array of magnetic field sensors mounted on an object moving in a first direction. The magnetic field sensors in the array are positioned a known distance apart along a second direction different from the first direction. Each sensor in the array detects the magnetic field signal along the first direction and generates a magnetic field signal corresponding to the intensity of the detected field. The computer reads the magnetic field signals from the sensors and determines a magnetic field peak in the first direction for each sensor. The magnetic field peaks are used to calculate a first slope of a first line between a first data point for a first sensor and a second data point for a second sensor. The peaks are next used to calculate a second slope of a second line between the first data point for the first sensor and a third data point for at least one third sensor. Then the processing module evaluates the first slope, the second slope and the peak data using the calibrated slope to determine the distance of the object from the tape along the second direction.
In a sixth embodiment, the present invention is a computer readable medium encoded with a computer program arranged such that, when executed, causes the computer to receive input signals representative of a magnetic field signals generated by an array of magnetic field sensors. The sensor array is mounted on an object, such as a vehicle or robot, moving in a first direction. The magnetic field sensors in the array are aligned with one another and positioned a known distance apart along a second direction generally normal to the first direction. Each sensor detects a magnetic field along the first direction and generates a magnetic field signal corresponding to that location. The computer determines the magnetic field peaks for each sensor in the array. The computer then calculates a first slope of a first line between a data point for a first magnetic field sensor and a data point for a second magnetic field sensor, and calculates a second slope of a second line between the data point for the first magnetic field sensor and a data point for a third magnetic field sensor. The first slope and the second slope are evaluated to determine the location of the sensors in the array with respect to the magnetic field source.
If the slope data indicates that the sensors are positioned over the magnetic field source, an appropriate distance calculation algorithm is used to determine the distance of the object from the magnetic field source along the second direction.
If the slope data indicates that the sensors are not positioned over the magnetic field source, an appropriate distance calculation algorithm based on a lookup table stored in memory is used to determine the distance of the object from the magnetic field source along the second direction. Using the magnetic peak values determined in the first direction, the computer retrieves a pair of peak difference values and their corresponding calculated distance values from the lookup table stored in memory. An interpolation procedure between the pairs of peak difference values in the lookup table is used to determine, based on the position of the sensors in the array along the second direction, the distance of the object from the magnetic field source along the second direction.
The present invention is a simple detection system for determining the position of an object with respect to a magnetic signal. As environmental conditions change, the system may encounter noise, the magnetization of the magnetic field source in a particular area may change, or the distance of the magnetic field sensors from the magnetic field source may vary. The detection system of the present invention calculates distance by comparing magnetic field data from multiple sensors positioned at known locations. This comparison step removes common mode noise (noise seen by all sensors), such as that seen from railroad tracks and other metallic objects, since this noise is subtracted out in the distance calculation. Thus, the detection system of the present invention provides accurate distance information in noisy environment. In addition, the measurement of magnetic field intensity data from multiple sensors makes the distance calculation independent of the height of the sensors above the magnetic field source. The system designer thus has great flexibility in selecting a location for the sensors on the object to be tracked. Once this position is selected, if the height of the object above the magnetic field source is changed for any reason, the distance calculation remains accurate.
In the system of the present invention the distance of the object from the magnetic field source may be determined by detecting only a single component of the magnetic field generated by the source. Additional sensors are not required, which reduces the impact of noise and lowers costs compared to conventional detection systems.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.